Devices that reproduce sound signals recorded and transmitted in form of digital or analog signals using sound reproduction means such as speakers are widely known. Examples of such devices include television and/or radio receivers, audio devices, and loud-speakers. Most of the devices except for some loud-speakers for outdoor use are used indoor. A room is a space enclosed by walls, and thus sound wave signals outputted through a speaker is reflected each time the sound signal arrives at a wall surface. Accordingly, sound wave signals that arrive at ears are signals obtained by synthesis of direct waves that arrive at the respective ears directly from the speaker and corresponding reflected waves reflected on the wall surfaces. The strengths of reflected waves from wall surfaces vary depending on the distances to the wall surfaces, the materials of the wall surfaces, and the structures of the walls. For example, a flat wall surface made of a hard material such as concrete or tile provides a high reflectance, thereby yielding a strong reflected wave.
A representative of spaces enclosed by wall surfaces is a bathroom in a home. Reflected waves arrive from various directions and have delay times different depending on the lengths of paths therefor. Such reflected waves that arrive at ears are synthesized waves of a number of such reflected waves, and thus are recognized not as independent sounds but as sounds each including echoic sounds or muffled sounds. This is generally called as reverberation. It is known that stronger reverberation decreases more significantly the clearness of a sound, resulting in decrease in the recognition rate of the sound.
One method for preventing such decrease in sound clearness due to reverberation is a method of correcting an input sound signal at the portions including reverberation that affects human auditory recognition, and then reproducing the sound from a speaker. For example, Patent Literature 1 discloses, as pre-processing for correcting influence of reverberation, a method for calculating a modulated spectrum from an input signal, enhancing a specific band of the modulated spectrum, and then re-synthesizing the sound signal from the processed modulated spectrum. According to this method, it is possible to reduce the sound pressure of the original sound at the portions on which sound waves reflected on wall surfaces and the like are superimposed, and in particular, it is possible to correct the influence of the reverberation on the variation in the amplitude slope in the temporal direction of the sound signal, and to increase the clearness of the sound under a reverberant environment (See Patent Literature 1).
[Patent Literature 1]
Japanese Unexamined Laid-open Patent Publication No. 2001-100774